The rapidity of modern-day data processing imposes severe demands on the ability to produce a printout record at very high speed. Impact printing, in which permanently shaped character elements physically contact a recording medium, has been found to be too slow, too bulky, and too noisy for many applications. Thus, the industry has turned to other alternatives involving non-impact printing schemes using various techniques to cause a desired character to be formed on the recording medium. Some of these involve the use of electrostatic or magnetic fields to control the deposition of a visible character-forming substance, either solid (i.e., dry powder) or liquid (i.e., ink) on the medium which is usually paper. Other systems utilize electro-photographic or ionic systems in which an electron or ion beam impinges on the medium and causes a change in coloration at the point of impingement. Still another system employs a thermal image to achieve the desired shape coloration change. Of more recent import is a printing technique, called ink jet or ink bubble printing, in which tiny droplets of ink are electronically caused to impinge on a recording medium to form any selected character at any location at high speed, each character being made up of a plurality of such droplets or dots. The present invention relates to this kind of printing system.
In the co-pending application, Ser. No. 292,841, entitled THERMAL INK JET PRINTER, filed Aug. 14, 1981 now abandoned by John L. Vaught et al. and assigned to the instant assignee, an ink-on-demand printing system is described which utilizes an ink-containing capillary having an orifice from which ink is ejected. Located closely adjacent to this orifice is an ink-heating mechanism which may be a resistor located either within or adjacent to the capillary. Upon the application of a suitable current to the resistor, it is rapidly heated. A significant amount of thermal energy is transferred to the ink resulting in vaporization of a small portion of the ink adjacent the orifice and producing a bubble in the capillary. The formation of this bubble in turn creates a pressure wave which propels a single ink droplet from the orifice onto a nearby writing surface or recording medium. By properly selecting the location of the ink-heating mechanism with respect to the orifice and with careful control of the energy transfer from the heating mechanism to the ink, the ink bubble will quickly collapse on or near the ink-heating mechanism before any vapor escapes from the orifice.
Thermal ink jet printheads may comprise a type in which the resistors are located on a substrate support member which is affixed to and aligned with a separate orifice plate with each orifice being positioned to cooperate with a discrete resistor in forming and ejecting an ink droplet. Separate barriers or hydraulic separators may also be provided as discrete components between the substrate and the orifice plate. Typical of this type of printhead structure is that shown and described in the co-pending application of Buck et al., Ser. No. 490,754, now U.S. Pat. No. 4,500,895 filed on May 2, 1983 and entitled DISPOSABLE INK JET HEAD.
In another type of printhead the resistors for each orifice may be actually formed on the orifice plate itself as integral parts thereof. This form of thermal ink jet head is shown and described in the co-pending application of Cloutier et al., Ser. No. 443,972, filed on Nov. 23, 1982 entitled ORIFICE PLATE/RESISTOR COMBINATION. In another co-pending application of Cloutier et al., Ser. No. 443,980, filed on the same date now U.S. Pat. No. 4,528,577 entitled INK JET ORIFICE PLATE HAVING INTEGRAL SEPARATORS, the hydraulic separators are also shown as integral with the orifice plate. The present invention relates particularly to a printhead structure which, in the preferred embodiment thereof, has the hydraulic separators formed as an integral part of the orifice plate, while the resistors are formed on a substrate member. The invention may, however, be utilized to advantage with structures in which the resistors are formed on the printhead orifice plate as well as any other type of ink jet printer where ink droplets or bubbles may be ejected from orifices by other than by the use of resistors. Typical of such other systems is that described and shown in U.S. Pat. No. 3,832,579 entitled POST DROPLET EJECTING SYSTEM wherein ink is ejected from a nozzle by means of a piezo-electric transducer. Still another system is described in U.S. Pat. No. 3,179,042 entitled SUDDEN STEAM PRINTER wherein electric current is passed directly through the ink itself which are contained in a number of tubes. Because of the high resistance of the ink, it is heated so that the portion in the tube thereof is expelled.
In ink jet printheads, and particularly the type to which the present invention relates, a phenomenon, commonly called "cross-talk", is encountered in which ink is ejected by the printhead from an orifice whose respective resistor has not been energized. This phenomenon arises when enough ink is pumped out of a non-fired orifice by the additive pumping action of previously fired resistors in the printhead. This pumping action causes the fluid to break free of the orifice plate in the nonfired orifices and land on the paper being printed. A line of text printed by such a head encountering this phenomenon will exhibit a random sprinkling of ink droplets superimposed on the text, seriously degrading the quality of the printing. In instances where all the resistors are being fired, an orifice-to-orifice consistency problem has been observed. Here the problem appears as a horizontal "banding" in which a variation in the print density in a block of fully-dense graphics occurs. It has been determined that the character of such banding results from the firing order of the resistors in the head and is caused by the fluid flow patterns in the head which are created in turn by the expansion and collapse of the vapor bubbles. These fluid flow patterns interfere either constructively or destructively with further firings of resistors in such a way as to alter the volume of fluid ejected by one particular orifice in a systematic way. While this effect can be reduced to some extent by prudent selection of the resistor firing order and the firing repetition rate, it is difficult to completely eliminate the problem by this route. The effect of firing order on print consistency is so great that it is possible to almost completely inhibit the ability of one orifice to eject an ink droplet when desired by timing the firing of its neighboring resistors so that collapse coincides with the other orifice's bubble expansion. By the basic rules of hydraulics the principal cause of the two problems described hereinabove is the noncompliant coupling of the fluid in any one orifice with the fluid in all the other orifices in the head. It is, therefore, highly desirable, and an objective of the present invention, to accomplish the decoupling of the dynamics of fluid motion in and near each individual orifice so that the bubble explosion, collapse and orifice refill processes occurring at one nozzle will not perturb those processes at other nozzles in the head. These problems may also be viewed as resulting from the difficulty in precisely controlling the energy imparted to each droplet so that upon ejection from one orifice, hydraulic energy excesses are dissipated through adjacent orifices.
Solutions to this "cross-talk" problem have been sought in various ways. For example, in the aforementioned pending application of Vaught et al., physical barriers between resistor/orifice pairs are provided. In the co-pending application of Tacklind, Ser. No. 419,658, filed Sept. 20, 1982 entitled METHOD AND APPARATUS FOR ELIMINATING ACOUSTIC CROSS-TALK and assigned to the instant assignee, a pattern-generating or multiplexing system for energizing the various resistors is disclosed. Orifice menisci null times are determined at which the effect of a previously ejected ink droplet will have little or no influence on subsequent ejections from other orifices. In U.S. Pat. No. 4,334,234 to Shirato et al. entitled LIQUID DROPLET FORMING APPARATUS, another solution is taught wherein communicating ports are provided between the actuating chamber (i.e., the particular cavity adjacent to an orifice for directly supplying ink to the orifice) and an intermediate ink chamber, the ratio of the area of the region of the inside wall surface of the intermediate chamber to the total opening area of the communicating ports is 50-300. In U.S. Pat. No. 4,338,611 to Eida et al. for liquid jet recording head, the printhead is constructed so that the following dimensional relationship is established: EQU 1/100=a/b=1/2
when the length from the orifice to the inlet port is L; the length of the energy acting zone is 1; the length of the orifice to the energy acting zone is a; and the length from the inlet port to the energy acting is b. L is held to be not less than 0.1 mm and not more than 5 mm and 1 is not less than 10 .mu.m and not more than 800 .mu.m.
The solution of the Shirato et al. and the Eida et al. patents attempts to decouple adjacent orifices by a manifolding technique to isolate neighboring orifices which are supplied with ink from a common ink source through individual feed tubes (ports). As can be seen, the length of these feed tubes is carefully chosen so that the inertia of ink entrained within a tube is sufficient to prevent large scale fluid displacements back into the supply line or feed tube (and hence to other feed tubes) when an ink droplet is ejected. The inertial isolation of orifices in this manner has several disadvantages. First, the extra feed tube length required to accomplish sufficient inertial isolation introduces an excessive fluid drag in the ink supply to the orifices, slowing down the rate at which they can be refilled after droplet ejection. Furthermore, the inertia of the entrained fluid in the feed tubes must be overcome in order to refill the orifices after ink ejection, since the inertia is, in effect, in series with the fluid circuit connecting the orifices with their supply of ink. This further restricts the rate at which the orifices can be refilled and hence further limits how fast the orifices can be repetitively operated (or "fired").
In the co-pending application of Allen et al., Ser. No. 490,753 filed May 2, 1983 entitled FLUIDIC TUNING OF IMPULSIVE JET DEVICES USING PASSIVE ORIFICES, and assigned to the instant assignee, another solution to cross-talk is described. In this approach the orifice plate is provided with "passive" or non-firing openings of various sizes and shapes. These non-firing openings are provided in the orifice plate adjacent to the active or firing orifices which are taught to be of the order of 0.003 inches (about 77 microns) in diameter. The diameter of the passive or non-firing openings is said to be of the order of the diameter of the firing orifices (thus being about 77 microns). In the co-pending application of Vaught, Ser. No. 490,684 filed May 2, 1983 entitled IMPULSE JET DEVICE HAVING INCREASED REPETITION RATE, assigned to the instant assignee, the firing orifices and the passive non-firing orifices are disclosed as having diameters on the order of 50 microns.